This invention was made with United States Government support under Contract No. F33657-99-D-0028 awarded by the Department of the Air Force. The United States Government has certain rights in this invention.
Most microwave filters built using microstrip transmission lines have a tendency of not suppressing 2nd, 3rd and 4th harmonic signals. Traditionally, the way to solve this problem is to add a lowpass filter at the two ends of a bandpass filter. Physically, this makes the filter structure bigger. Electrically, using lowpass filters increase signal loss, and the suppression of the harmonics for the most part is not as good as desired.